Permian igneous clasts from the Matzitzi Formation, southern Mexico: isotopic constraints on the final amalgamation of Pangaea

The tectonic reconstructions of southern Mexico during Late Paleozoic–Middle Triassic time are related to the final stages of Pangaea assembly. Petrographic analysis combined with U–Pb and Hf isotopic analyses of zircon grains belonging to volcanic conglomerate clasts from the Matzitzi Formation in southern Mexico allow characterization of the magmatism at the end of Pangaea accretion. Volcanic pebbles from the Matzitzi Formation have felsic to intermediate compositions and ages ranging from 269 to 285 Ma, which indicate the erosion of a Permian volcanic source. The ɛHf zircon values show two groups with different isotopic signatures. Intermediate hypabyssal samples have εHf(t) values ranging from −7.8 to +0.2 and two-step model ages from 1.6 to 1.7 Ga, suggesting late Paleoproterozoic to Mesoproterozoic crustal residence times. Felsic volcanic samples have εHf(t) values ranging from −3.5 to +8.8 and crustal model ages ranging from 0.7 to 1.5 Ga, suggesting Neo-Mesoproterozoic crustal sources and a juvenile depleted mantle source. Grenvillian grains in volcanic clasts, as well as their isotopic signatures, suggest that the volcanic rocks were emplaced over Mesoproterozoic Oaxaquia-type crust and are related to the Permian–Carboniferous plutonic arc in southern Mexico, which formed along the northwestern Gondwana margin.